Astronomers watched as a star passed within 12 billion miles of the black hole at the center of the Milky Way and reached a speed of 15 million miles per hour.

The closest supermassive black hole to Earth is a mere 26,000 light years away, situated right in the middle of our galaxy. In May, astronomers at the European South Observatory (ESO) in Chile observed a star called S2 make its closest pass by the black hole and got to observe how the intense gravitational field affected the star’s motion. By studying S2 while it made its pass, the astronomers were able to confirm a crucial prediction of Einstein’s general theory of relativity by turning nature into a readymade gravitational laboratory.

With a mass four million times greater than our Sun, the Milky Way’s largest supermassive black hole holds the title for the strongest gravitational field in the galaxy. Extreme conditions are the norm here and any stars that get caught in its gravitational field will orbit the black hole at millions of miles per hour. These extreme gravity conditions make the black hole an ideal place to test theories of gravitational physics, such as Einstein’s theory of general relativity.

According to Einstein’s theory, strong gravitational fields should stretch out the wavelength of light from stars within the field. This phenomenon is known as gravitational redshift because the longer wavelengths make the starlight appear redder.

When S2 made its closest pass in May it was 12.4 billion miles from the black hole and moving at over 15 million miles per hour, or bout 3 percent of the speed of light. During the pass, ESO astronomers observed a strong redshift in S2’s light. The redshift agreed precisely with the values predicted by Einstein’s general theory of relativity.

A simulation of stellar orbits around the supermassive black hole in the center of the galaxy. Image: ESO

ESO astronomers have been observing the Milky Way’s black hole for 26 years, but only recently have their instruments been sensitive enough to make the kinds of observations necessary to test Einstein’s theory this way. S2 only orbits the black hole once every 16 years, so this time around the astronomers wanted to make sure they were ready to take advantage of the ready-made gravitational lab provided by nature.

The ESO has four main telescopes considered to make up the most advanced telescope array in the world. Each one is outfitted with a mirror nearly 27 feet in diameter. The recent research was only possible due to a new suite of hyper-sensitive devices used by the telescopes. An instrument called SINFONI was used to measure the velocity of the star and another instrument called GRAVITY made high resolution images of S2 each night to see its motion and plot the star’s orbit.

“This is the second time that we have observed the close passage of S2 around the black hole in our galactic centre,” Reinhard Genzel, a physicist at the Max Planck Institute for Extraterrestrial Physics, said in a statement. “But this time, because of much improved instrumentation, we were able to observe the star with unprecedented resolution. We have been preparing intensely for this event over several years, as we wanted to make the most of this unique opportunity to observe general relativistic effects.”